Field of the Invention
The present invention relates to an electromagnetic shielding film, a flexible printed wiring board provided with the electromagnetic shielding film, an electronic device provided with the flexible printed wiring board, and a method for forming the same.
Priority is claimed on Japanese Patent Application No. 2013-139991, Jul. 3, 2013, the content of which is incorporated herein by reference.
Description of the Related Art
An electromagnetic shielding film is provided on a surface of a flexible printed wiring board in order to shield an electromagnetic wave noise generated from the flexible printed wiring board and an electromagnetic wave noise generated from outside.
For example, the following flexible printed wiring board is proposed as a flexible printed wiring board with an electromagnetic wave shielding film.
(1) Japanese Patent Publication No. 4201548 discloses a flexible printed wiring board provided with an electromagnetic shielding film 100 which includes a flexible printed wiring board 120 in which a printed circuit 124 and an insulating film 126 are provided on at least one surface of a base film 122 and an electromagnetic shielding film 110 in which an electromagnetic shielding layer 118 including a metal thin film layer 114 and a conductive adhesive layer 116 is provided on a surface of a cover film 112 as illustrated in FIG. 9. The conductive adhesive layer 116 is adhered to a surface of the insulating film 126 and a surface of a ground circuit 124a in an opening portion 126a of the insulating film 126.
Recently, in order to reliably ground the electromagnetic shielding film, an electromagnetic shielding layer of the electromagnetic shielding film has been electrically connected to an external conductor other than the flexible printed wiring board, which is a housing at least a portion of which has conductivity, a conductor which functions as ground, or the like, and the external conductor has been used as ground of the electromagnetic shielding film.
For example, the following flexible printed wiring board is proposed as a flexible printed wiring board with the electromagnetic wave shielding film which can electrically connects the electromagnetic shielding layer of the electromagnetic shielding film to the external conductor.
(2) Japanese Patent Publication No. 4201548 discloses the flexible printed wiring board with the electromagnetic wave shielding film illustrated in FIG. 9 in which a ground part 130 including a conductive bump 132 (protrusion object) which penetrates the cover film 112 is provided on a surface of the cover film 112 while an adhesive layer 134 is interposed between the ground part 130 and the cover film 112 as illustrated in FIG. 10.
However, according to the flexible printed wiring board with the electromagnetic wave shielding film described in (2), in order to secure the electrical connection between the electromagnetic shielding layer of the electromagnetic shielding film and the external conductor, the conductive bump penetrating the cover film should be provided. Therefore, electrically connecting the electromagnetic shield layer and the external conductor is complicated. Additionally, there is a possibility that the conductive bump may not penetrate the cover film or that the conductive bump may not reach the electromagnetic shielding layer if the conductive bump penetrates the cover film. Therefore, the electrical connection between the electromagnetic shielding layer and the external conductor is uncertain.
For example, the following flexible printed wiring board is proposed as an electromagnetically shielded flexible printed wiring board which can electrically connect the electromagnetic shielding layer of the electromagnetic shielding film to the external conductor without using the conductive bump.
(3) Japanese Patent Publication No. 4575189 discloses the electromagnetically shielded flexible printed wiring board 102 as illustrated in FIG. 11. The cover film 112 is eliminated from the flexible printed wiring board with the electromagnetic wave shielding film 100 illustrated in FIG. 9, and an anisotropic conductive adhesive sheet 140 which has conductivity in a thickness direction by conductive particles 142 is provided on a surface of the metal thin film layer 114.
The flexible printed wiring board may be required for a thermal resistance since an electronic component, a terminal, and the like are soldered with the flexible printed wiring board. When the electromagnetically shielded flexible printed wiring board described in (3) is required for the thermal resistance, a polyimide film which has superior thermal resistance is used for the base film and the insulating film, and a cured material of a thermosetting adhesive with the superior thermal resistance such as an epoxy resin is used for the conductive adhesive layer. Further, an anisotropic conductive adhesive sheet without the heat resistance cannot be provided to the electromagnetically shielded flexible printed wiring board in advance. When the flexible printed wiring board with the electromagnetic wave shielding layer without the anisotropic conductive adhesive sheet is adhered to the external conductor, the anisotropic conductive adhesive sheet is provided between the flexible printed wiring board with the electromagnetic wave shielding layer and the external conductor.
Therefore, it is only necessary to provide the electromagnetic shielding layer on the surface of the flexible printed wiring board. As a method for providing only the electromagnetic shielding layer on the flexible printed wiring board, it is considered that, as shown in FIG. 12, an electromagnetic shielding layer transferring film 150 in which the electromagnetic shielding layer 118 including the metal thin film 114 and the conductive adhesive layer 116 are provided on a surface of a peeling film 152 is adhered to the surface of the flexible printed wiring board 120, and the peeling film 152 is peeled after curing the conductive adhesive layer 116.
However, the following phenomenon may occur in the above-described method.
i) It is difficult to form the metal thin film 114 on the surface of the peeling film 152 by a vapor deposition.
ii) Since the adhesion of the peeling film 152 and the metal thin film 114 is strong, the metal thin film 114 is easily peeled along with the peeling film 152 when the peeling film 152 is peeled.
iii) Since the cured conductive adhesive layer 116 is fragile, the electromagnetic shielding layer 118 easily breaks when the peeling film 152 is peeled.
In order to form the flexible printed wiring board with electromagnetic shielding layer with a favorable yield, practically, it is necessary to directly form the conductive adhesive layer on the flexible printed wiring board by a wet process and further form the metal thin film thereover.
In view of simplicity of forming, a method in which the independent electromagnetic shielding film is prepared, and the electromagnetic shielding film is adhered to the flexible printed wiring board by a dry process is desired.